In all tire and wheel rim assemblies safety is of paramount importance. When pneumatic tires are subjected to loss of air pressure the safety of the vehicle is greatly reduced and, with the advent of tires which may be run for substantial distances even when punctured so that the normal required air pressure is not retained, this problem has become the subject of extensive development.
The primary reason for the unsafe condition when the tire is deflated or substantially deflated is tire bead dislodgement from the bead seats when a side force is applied.
In conventional tire and wheel rim assemblies the tire beads are retained on their respective bead seats by means of the internal air pressure and the frictional restraint due to the compression in the elastomer under the bead wire when fitted onto the tapered seat of the wheel rim.
Lowering the air pressure in the tire lessens the retention force from the internal air pressure and eventually at a sufficiently low internal pressure the assembly reaches an unsafe condition in which the tire beads may be displaced from their seats by a sideways force such as is generated in an accident avoidance maneuver or even in the case of steering the vehicle to effect a lane change to find a place where a deflated tire may be removed and a spare fitted.
The U.S. Department of Transportation has standard tests for automobile tires which include FMVSS 110 and FMVSS 109.
FMVSS 110 requires that the tire is retained in position on the wheel rim while the vehicle is stopped in a straight line under controlled braking from a speed of 60 mph at which speed the tire is suddenly completely deflated. FMVSS 109 Section S 5.2 relates to a tire bead unseating test carried out statically by means of applying a point load by an anvil to a point on the mid-sidewall of a tire and increasing the load until unseating occurs.
Neither test applies a dynamic side force condition which occurs in use of a vehicle when a puncture causes deflation of the tire and steering is required.
The European Motor Industry uses a variety of tests to check bead dislodgement. A typical test is carried out by testing a tire and wheel rim assembly as the outer front wheel, i.e. left-hand or near side, in a right-hand J turn test at 25 mph. The test comprises straight running at 25 mph and then the sudden application of full steering lock. The test is repeated at progressively reducing tire inflation pressures until bead dislodgement occurs. Usually the pressure reduction steps are 2 psi. Typical production radial ply car tires normally dislodge a bead at air pressures of the order of 5-5 psi in such a test.
Nevertheless, dislodgement of a tire bead from its seat when it does occur, may seriously affect vehicle control. In the case of wheel rims which include a well to allow tire fitting there is generally a grave danger of complete separation of the tire from its wheel rim. This action or any contact of the wheel rim with the ground may have serious consequences to the vehicle and occupants.
In the use of a vehicle, cornering generates sideways forces which displace the tread laterally with respect to the wheel rim. These forces are transmitted by the tire to the tire bead. In the region of the tire adjacent to the ground contact area axial forces (i.e., in the direction of the tire axis) and turning moments (i.e., about a circumferential line through the bead) are generated. In the absence of air pressure these forces may be sufficient to produce lifting of the heel of the bead reducing the frictional force between the bead base and the bead seat on the wheel rim which, in said deflated state of the tire, is the only force which retains the bead on its seat. As a result the bead moves down its tapered bead seat laterally inwardly of the wheel rim flange reducing the tension in the bead wire and, very rapidly, the residual bead retaining force becomes less than the dislodging forces and the bead leaves its seat and falls into the well.
Previous attempts to solve this problem have centered around the use of a wheel rim which does not have a well. Such flat-based wheel rims obviate the danger of tire wheel separation but have the disadvantage that the tire beads, once unseated as above, are usually able to move axially between the two spaced-apart flanges. Thus the sideways force which can therefore be transmitted between the wheel and the ground may change suddenly from zero when the bead is moving across the wheel rim to a maximum when both beads are together against one flange. The change may, in the extreme, cause loss of control of the vehicle.
This is equally true for a well based wheel rim having a filling device, a wheel rim which has a fitting well closed by crimping after tire fitting or a divided wheel rim assembly.
The divided wheel system requires several extra components with consequent disadvantages in sealing the air chamber, increased cost, increased weight and increased complexity for servicing and repair and thus increased danger of incorrect assembly. The well filling system also increases the assembly weight, cost and complicates both fitting and servicing even though basically a one-piece wheel is used. Furthermore when the tire is fitted and inflated there is no way of checking whether correct and therefore safe assembly has been carried out.
None of these systems avoids the problem of side force transmission when a bead moves axially across the wheel rim but proposals to do this are known. For example U.K. Pat. Specification No. 222,768 discloses a bead spacer ring which comprises a rigid, circumferentially extending ring which fits between the tire beads to fill the space between them and prevent their inward movement. Such devices, although basically effective, add yet further components, weight and cost to the assembly. They are complicated and difficult to fit and service.
Another published modification to a divided flat based wheel rim is shown in U.S. Pat. No. 3,857,429 in which the two wheel rim components are divided adjacent to the outboard bead seat and shaped so as to form a notch. A rubber toe is provided on the tire bead which rests freely in said notch. The assembly is still a complex divided wheel rim assembly and uses a flat based wheel rim.
Another system shown in U.K. Pat. No. 890,959 provides a rubber covered fabric reinforced extension of the tire bead which is clamped between the two components of a divided wheel rim. The clamping is used to seal the assembly and while it may retain the bead it would not prevent some bead movement when deflated. The assembly is also subject to the general disadvantages of divided wheel rims.
A one-piece flat based wheel rim has also been proposed in U.K. Pat. No. 1,348,891 in which a well is provided for tire fitting which well is subsequently closed by permanently crimping the wheel rim axially so as to close the fitting well. In such an assembly it is necessary to destroy the wheel rim to remove the tire for service.
Another published system provides a well for tyre fitting in the region of one bead seat on an otherwise flat based wheel rim and when the tire has been fitted over the flanges both beads are held against the flange furthest from the well while a well filling ring is inserted to provide the second bead seat. Such an assembly is shown in U.S. Pat. No. 3,884,286.
The problems and complexities of the above-mentioned systems are in general unacceptable to the U.S. automobile manufacturers who currently fit tubeless tires on to wheel rims using automatic fitting machines. Usually two operations are carried out. Firstly the tire is placed over the wheel rim and a pair of rollers beginning at a common point press both beads together over the flange into the well of the wheel rim. The rollers then move in opposite directions around the periphery of the wheel rim flange thus simultaneously fitting both beads over the flange. Secondly, the tire/wheel rim assembly is inflated by an `explosion` fitting operation which almost instantaneously inflates the tire and forces both beads home onto their respective bead seats.
Accordingly many attempts have been made to allow the use of normal well-based wheel rims. The simplest method is to provide in the wheel rim circumferentially extending humps formed adjacent to the bead as shown, for example, in the drawings of U.S. Pat. No. 3,540,510. In order that the tire beads may be fitted to their respective bead seats upon inflation such humps can be 1.7 mm in height relative to the bead seat toe diameter but in any case must be limited in height to prevent tire damage on fitting. The sideways forces generated by the road when the vehicle is steered with the tire deflated greatly exceed the forces generated by the inflation pressure and thus the beads can be moved back over the humps.
One-piece wheels have been modified to provide high humps by means of radially movable stops but such devices again add complexity, cost, weight, and are also difficult to make air-tight. The apertures required produce serious points of weakness in the wheel rim itself.
More recently it has been proposed in U.S. Pat. No. 3,951,192 to provide a bead latch in which an axial extension is formed on the outside of the tire sidewalls which is shaped to engage around the wheel rim flange so as to resist bead movement. However, sideways forces rotate the bead with resulting heel lifting.
The object of the present invention is to provide a secure bead retention system which overcomes all the aforementioned problems, allows the use of a one-piece wheel rim and which allows for automatic assembly using conventional techniques.
The inventors have investigated the phenomena of the forces involved in dislodging the beads of a tire from a wheel rim when on a vehicle. They have also investigated the forces involved in fitting tires to and removing tires from wheel rims. They have found that the road generated forces involved in tire bead dislodgement are quite different from the forces involved in the removal of a tire from a wheel rim with the wheel tire/rim removed from the vehicle for tire replacement or repair.
Accordingly the inventors utilize the difference between the road/tire generated dislodgement forces and the tire removal forces to provide a bead lock which meets the object.